The present invention relates, generally, to power control in radio transmitters and, more particularly, to transmitters used in telecommunications systems, such as used in mobile terminals.
Because the number of users of mobile terminals has increased tremendously, there have been increased efforts to increase the capacity of mobile networks. As one possible solution, the Code Division Multiple Access (CDMA) technology has been under discussion for years. In Europe, the UMTS research work has mostly concentrated on the wide band CDMA technology. On the other hand, there also has been a multitude of other research efforts relating to further development of the already existing Global System for Mobile communications (GSM) technologies. For example, Enhanced Data rates for GSM Evolution (EDGE) is currently coming to market.
In the GSM, the Gaussian Minimum Shift Keying (GMSK) modulation is used. On the contrary, in the EDGE, the 8PSK modulation scheme will be used. A remarkable difference between these two modulation schemes is that the 8PSK modulation is a nonconstant envelope modulation scheme, whereas the GMSK is a constant envelope modulation scheme. As such, that in the 8PSK modulation, some part of the information is contained in the amplitude of the envelope of the transmitted signal and some part is contained in the phase of the transmitted signal. The amplitude modulation makes it more difficult to control the power of the transmitter. The main reason for the difficulty is that the varying amplitude causes variations in the peak signal power. The amplitude depends on the signals that are sent, and the peak power varies between time slots or data bursts that are sent with the same average output power.
Some solutions for controlling the power output of a radio frequency transmitter can be found in prior art. The European patent application 0 688 109 A2 describes an output power control arrangement for a burst transmitter. In '109, the idea is that the output power envelope shaping of the prior burst to be transmitted is controlled in a feedback loop producing a control signal. When the signal is to be transmitted using amplitude modulation, the feedback loop is switched off for the period of information transfer, but closed at other times. Conversely, for bursts to be transmitted with a constant envelope modulation the loop is not switched off.
International patent application WO 00/33477 presents a peak to average power ratio limiting apparatus to be used in a CDMA network. Because the air-interface bandwidth is shared between multiple users, the compound signals may have high peaks to average power ratio to be processed by the transceiver. Because compound signals are input to the power amplifier of a mobile terminal, the amplifier usually has a poor power efficiency. In '477, the system has a squared envelope magnitude predictor equivalent to a power estimation apparatus that estimates the squared magnitude of the modulated CDMA waveform envelope. The envelope is finally formed by the base band signal after quadruple modulation. The predicting requires excessive averaging in order to find the average power through the burst, which results in a narrow resolution bandwidth.
A third prior art power output control system is presented in published UK patent application GB 2 368 208 A. The system measures information relating to statistical variations in the amplitude of the information signal to be transmitted. The statistical variations are used to control a gain value of the radio frequency transmitter. The statistical amplitude variation of the non-constant envelope modulation is taken into account and compensated. This requires excessive computation as well.
It is an object of the present invention, therefore, to provide a solution by which it is possible to bring about a system with which the output power of a radio transmitter can be controlled in a simple and reliable manner and the component count reduced by use of software control.